The maintenance of an intact genome is of crucial importance to every organism. The individual cell in a multicellular eukaryotic organism possesses sophisticated and intricate mechanisms to properly respond to DNA damage. Such mechanisms repair damaged DNA or trigger programmed cell death (apoptosis). In response to DNA damage, checkpoint kinases are thought to be intimately involved in these processes. These kinases are activated by upstream proteins, such as ATM (ataxia-telangiectasia mutated) and ATR (ataxia-telangiectasia mutated and rad3-related), and in turn trigger cell cycle arrest by inhibition of proteins such as Cdc25A or Cdc25C. The checkpoint kinases may also modulate the activity of other proteins that are thought to be involved in DNA repair and programmed cell death. Examples of such proteins are BRCA1 and p53.
The checkpoint kinase Cds1 (in man also known as Chk2) is conserved from yeast to man. A human homolog of the Schizosaccharomyces pombe Cds1 gene has been described (Tominaga, K. et al. J. Biol. Chem. 1999, 274(44), 31463-31467; Matsouka, S. et al. Science 1998, 282, 1893-1897; Blasina, A. et al. Curr. Biol. 1999, 9(1), 1-10). Human Cds1 was rapidly activated by phosphorylation in response to DNA damage in both normal cells and in p53-deficient cancer cells. High levels of hCds1 were observed in p53-deficient cells. In human cells Cds1 has been implicated in the regulation by phosphorylation of proteins such as p53, BRCA1, Cdc25A, and Cdc25C (See: Lee, J.-S. et al. Nature 2000, 404, 201-204; Falck, J. et al. Nature 2001, 410, 842-847; and Buscemi, G. et al. Mol. Cell. Biol. 2001, 21(15), 5214-5221). As described below, inhibition of Cds1 offers two strategies for improving the effectiveness of DNA-damaging cancer treatments.
Cancer cells are often deficient in the mechanisms responsible for maintaining an intact genome. In particular, they have often lost proper p53 function, which generally correlates with the progression of a tumor to a more aggressive state, such as the progression from a pre-invasive to invasive stage of colon cancer, or from a low grade to a high grade astrocytoma. Between 30% and 70% of all subtypes of tumors have a point mutation in one of the two p53 gene copies and have lost the other allele. P53-deficient cells are generally more resistant to radiation. It is thought that the lack of initiation of programmed cell death in cancer cells may render such cells less sensitive to DNA-damaging cancer treatments. The transcription factor p53 is of importance not only for the initiation of programmed cell death, but also in cell cycle arrest. Loss of p53 function may therefore leave cancer cells with limited protection against insult to the genome. Further disruption of DNA damage repair and cell cycle arrest by inhibition of kinases such as Cds1 could then render cancer cells unable to survive after DNA damage. Therefore, inhibition of Cds1 could, by removing the remaining components of DNA damage repair, render the cancer cells more susceptible to treatments such as chemical DNA-damaging agents or ionizing radiation.
Normal cells, on the other hand, have an intact p53 system, and will often undergo apoptosis in response to DNA-damaging treatments at a much lower dose than that required to kill cancer cells. Therefore, in such situations, normal cells will be at a disadvantage compared to cancer cells, and cancer treatments often have to be discontinued due to serious side effects caused by loss of normal cells before the cancer has been eradicated. Inhibition of Cds1, which would prevent this kinase from phosphorylating and thereby stabilizing p53, could therefore protect normal cells from the effects of ionizing radiation or DNA-damaging chemotherapeutics while still allowing these agents to be effective against p53-deficient cancer cells. This would have the effect of increasing the therapeutic potential of these agents. This view is supported by studies of mice deficient in Cds1 (See: Hirao, A. et al. Mol. Cell. Biol. 2002, 22(18), 6521-6532; Takai, H. et al. EMBO J. 2002, 21(19), 5195-5205; WO 01/98465 A1 Chugai Seiyaku Kabushiki Kaisha, Dec. 27, 2001). These animals showed increased resistance to the apoptosis caused by ionizing radiation over their wild-type counterparts. For example, it was shown that these animals were protected from apoptosis of intestinal cells, hair follicle cells, cells of the CNS, and thymus cells relative to their wild-type counterparts when treated with ionizing radiation. Cds1 knockout animals also showed increased survival when exposed to ionizing radiation. It is therefore logical to assume that chemical inhibitors of Cds1 would have therapeutic potential in the protection of patients from the deleterious side effects of radiation or DNA-damaging chemotherapeutics.
A p53 deficient tumor is a tumor wherein the functions mediated by p53 are lacking or suppressed due to genetic mutations in the gene encoding p53 or through deficiencies or disregulation of proteins that modulate p53 expression levels and function. Examples of such proteins are MDM2 and p14(ARF).
Additional examples of cell cycle checkpoint modulators in development include UCN-01 (CAS 112953-11-4), UCN-02, KW-2401, NSC-638850 (Kyowa Hakko/National Cancer Institute) and SB-218078 (CAS 135897-06-2) (SmithKline Beecham).
Additional relevant publications include DE 0148431 (T 7570), WO 01/21771 A2, WO 02/072090 A1, WO 03/011219 A2, and White, A. W. et al. J. Med. Chem. 2000, 43(22), 4084-4097.
The features and advantages of the invention are apparent to one of ordinary skill in the art. Based on this disclosure, including the summary, detailed description, background, examples, and claims, one of ordinary skill in the art will be able to make modifications and adaptations to various conditions and usages. Publications described herein are incorporated by reference in their entirety.